1. Field of the Invention
This invention relates to a flexible tube for an endoscope and an endoscope equipped with the flexible tube.
2. Description of the Prior Art
Endoscopes for medial use or industrial use are equipped with flexible tubes (flexible tubes for endoscopes). Generally, the conventional flexible tubes for endoscopes have a structure which includes an elongated tubular core obtained by covering the outer periphery of aspiral coil with a reticular tube (braided member) and an outer cover formed of a synthetic resin or the like and provided over the outer periphery of the tubular core.
In endoscopic examination, the flexible tube for an endoscope is inserted along the body cavity to a deep part such as the stomach, duodenum, small intestine, and large intestine with being bent appropriately according to the shape of the path. In order to perform the inserting operation easily and reliably, it is necessary for the flexible tube that a push-in force applied to the proximal end (an end which is close to the operator) of the flexible tube is fully transmitted to its distal end. In other words, an flexible tube for an endoscope in which buckling is likely to occur has a poor operability. Here, “buckling” means a state that the push-in force applied at the proximal end of the flexible tube can not be fully transmitted to the distal end thereof because the push-in force is partially absorbed by a bent part in the flexible tube (where the buckling occurs). In order to avoid the occurrence of such buckling, it is necessary for a flexible tube for an endoscope to have sufficient flexibility so as to be able to withstand applied bending forces that would cause buckling. Further, the outer cover must be firmly attached or adhered to the tubular core since buckling is liable to occur at areas where the outer cover is peeled off from the tubular core.
Furthermore, in order to perform the inserting operation easily and reliably, It is also necessary for a flexible tube for an endoscope that when a rotational force (a twist) is applied to the proximal end thereof, the rotation is fully transmitted to the distal end thereof without being absorbed somewhere along the flexible tube. In other word, a flexible tube for an endoscope is also required to have satisfactory rotation followability at the distal end thereof for rotational force applied at the proximal end.
One example of such flexible tubes is disclosed in Japanese Examined Patent Publication No. Hei 5-50287, in which an outer cover of a flexible tube for an endoscope is constructed from a double layer structure comprised of an outer layer made of a material having good flexibility and an inner layer made of a material having good resiliency, thereby improving elasticity or resiliency of the flexible tube for an endoscope as a whole.
However, in the above-mentioned prior art, the adhesion (bonding strength) between the outer cover and the core has been left out of consideration. Therefore, in the flexible tube of the prior art, there is a case that the outer cover is peeled off from the core (that is, floating-up of the outer cover occurs) because of bending stress or twisting stress is exerted frequently at an area in the vicinity of a boundary surface between the outer cover and the core by repeatedly using the flexible tube, and thereby the resilience and the buckling withstanding property of the flexible tube are lowered. In other words, the prior art flexible tube involves a problem in its durability.